Understanding Grease Compatibility

"Is there a grease compatibility table? What are the parameters that control the compatibility of different types of grease thickeners?"

The decision to make a lubricant change within a machine should always be carefully evaluated. With greases, as opposed to oils, it is especially important to be cautious when switching from one lubricant type to another. Due to grease’s fundamental properties, it can be difficult to remove all of the old grease before introducing new grease to a machine in the same manner that oil can be drained from a machine to add new oil.

Because of this challenge, the decision to use a new grease in a machine must be managed with grease compatibility in mind. The chart below illustrates the compatibility between most grease types with three categories: compatible, borderline compatible and not compatible. Whether two greases are compatible will depend on the thickener type and base fluid as well as the resulting grease properties after the two grease types have been sufficiently mixed.

Grease compatibility can be complicated even to the average lubrication technician. One common mistake is when grease specifications are based on performance without considering composition. This can result in serious consequences if two different greases with the same performance criteria (such as high-temperature greases) are mixed together and their compositions are incompatible.

Another concern relates to the compatibility between polyurea and greases based on simple lithium soaps (and complex lithium soaps). Polyurea thickeners have a variety of possible compositions, with some perfectly compatible with lithium soaps and others quite incompatible.

When a change in grease type occurs, it is essential to test the mixture of the two greases. ASTM D6185-11 was developed to evaluate the compatibility of lubricating grease binary mixtures. This standard allows you to assess three specific performance measurements: dropping point, mechanical stability and consistency. Depending on the results, further testing may be necessary to ensure grease compatibility.

While the compatibility chart offers an effective way to determine general compatibility between two greases, performing grease compatibility tests may be preferred when critical applications are involved. In many cases, two greases designed for the same application can have detrimental characteristics when mixed together.

Working with grease suppliers should also be part of the compatibility decision process, as they may have solutions for the specific application affected by the change in grease type. By being thorough and cautious in your assessments, you can ensure extended machine life and avoid significant costs due to premature failures.